Hi.
Your case is suitable for using Generated Columns. So the age column become a calculated column, which value will depend on the birth column.
https://backendless.com/docs/rest/data_generated_columns.html
Try the functions:
Use NOW() or NOW(fraction_part) → returns ‘YYYY-MM-DD hh:mm:ss.fraction’
or
UnixTimestamp() or UnixTimestamp(from) → seconds since ‘1970-01-01 00:00:00’ UTC or from the argument